1. Technical Field
The present invention relates to an optical head for recording or reproducing information to or from an information recording medium such as an optical disc, an optical disc device comprising such an optical head, an information processing device comprising such an optical disc device, and an optical element with a diffraction grating for guiding a laser beam to a photodetector that creates an automatic power control signal for controlling an output of a light source.
2. Background Art
An optical head for recording or reproducing information to or from an information recording medium such as an optical disc comprises a front monitor sensor for detecting a part of the laser beam that is emitted from the light source in order to more accurately control the output of the laser beam that is emitted from the light source particularly during the recording process. The detection signal in the front monitor sensor is an APC (Automatic Power Control) signal. The APC signal is fed back to the control unit for controlling the output of the light source. The APC signal is used for controlling the output of the light source so that appropriate power that is required for the recording and/or reproduction of the information can be obtained.
Nevertheless, if the laser beam that is emitted from the light source is converted into parallel light with, for instance, a collimator lens or the like, the optical axis of the laser beam that is headed toward the front monitor sensor after passing through or being reflected by the planar beam splitter or the planar reflective mirror and the optical axis of the laser beam that is headed toward the front monitor sensor after being internally reflected by the planar beam splitter or the planar reflective mirror will mutually become parallel and cause interference. Consequently, the APC signal in the front monitor sensor will no longer be accurately proportionate to the output of the light source.
FIG. 17 is a diagram showing the condition of transmitted light and reflected light in a conventional planar beam splitter. For example, as shown in FIG. 17, parallel light P1 that is emitted from within the effective region of the collimator lens enters a planar beam splitter 105. Here, the parallel light P1 is split into reflected light R that was reflected by a first surface 105a and transmitted light T1 that passed through the first surface 105a and a second surface 105b and is headed toward the front monitor sensor. Here, parallel light P2 that was emitted from within a different effective region of the collimator lens passes through the first surface 105a, and is subsequently reflected by the second surface 105b. Further, the parallel light P2 that was reflected by the second surface 105b is reflected by the first surface 105a, subsequently passes through the second surface 105b, and is emitted as the transmitted light T2.
Here, if the optical axis of the parallel light P1 and the optical axis of the parallel light P2 are mutually parallel, and the first surface 105a and the second surface 105b of the planar beam splitter 105 are mutually parallel, the optical axis of the transmitted light T1 and the optical axis of the transmitted light T2 will become mutually parallel, and cause an interference in the effective region of the front monitor sensor. Thus, even if the output of the light source is changed linearly, the APC signal that was detected with the front monitor sensor and converted into an electrical signal will not change linearly.
As described above, in an optical configuration where parallel light is caused to enter the planar beam splitter or the planar reflective mirror in which the light incident plane and the light emission plane are parallel, it will become difficult to accurately control the output of the light source.
Thus, Patent Literature 1 describes an optical pickup for inhibiting the interference of the laser beam caused by the internal reflection with the planar beam splitter by adopting an optical configuration in which converging light or diverging light enters the planar beam splitter. In addition, Patent Literature 1 describes an optical pickup for inhibiting the interference of the laser beam caused by the internal reflection with the beam splitter by using a wedge-shaped beam splitter.
The conventional optical pickup described in Patent Literature 1 is now explained with reference to FIG. 18. FIG. 18 is a diagram showing a schematic configuration of a conventional optical pickup.
In FIG. 18, an optical pickup 150 is configured from first and second light sources 110, 120 for emitting light respectively having different wavelengths, a planar beam splitter 125, first and second collimator lenses 114, 124 disposed between the first and second light sources 110, 120 and the planar beam splitter 125, a front monitor sensor 126, a mirror 127 and an objective lens 129.
In the conventional optical pickup 150, the front monitor sensor 126 is used for detecting an APC signal. In the foregoing case, the first collimator lens 114 is disposed so as to convert the laser beam that is emitted as diverging light from the first light source 110 into converging light or diverging light. Specifically, the first collimator lens 114 is disposed in a state of being moved to a position that is closer to the first light source 110 or to a position that is farther away from the first light source 110 than a position of changing the laser beam that is emitted from the first light source 110 to parallel light.
For example, a case of converting the laser beam that is emitted from the first light source 110 into diverging light with the first collimator lens 114 is now explained with reference to FIG. 19. FIG. 19 is a diagram showing the condition of the transmitted light and the reflected light in the planar beam splitter of the conventional optical pickup shown in FIG. 18.
In FIG. 19, the planar beam splitter 125 includes a first surface 125a which the laser beam that was emitted from the first light source 110 enters, and a second surface 125b facing the first surface 125a. Diverging light Q3 that was emitted from the effective region of the first collimator lens 114 enters the planar beam splitter 125. Here, the diverging light Q3 is split into reflected light R that was reflected by the first surface 125a and transmitted light T3 that passed through the first surface 125a and the second surface 125b and is headed toward the front monitor sensor 126. Here, diverging light Q4 that was emitted within a different effective region of the first collimator lens 114 will be reflected by the second surface 125b after passing through the first surface 125a. In addition, the diverging light Q4 that was reflected by the second surface 125b is reflected by the first surface 125a, subsequently passes through the second surface 125b, and is emitted as transmitted light T4.
Here, since the diverging light Q3 and the diverging light Q4 are emitted from different effective regions of the first collimator lens 114, the optical axis of the diverging light Q3 and the optical axis of the diverging light Q4 will not be parallel. Accordingly, even if the first surface 125a and the second surface 125b of the planar beam splitter 125 are mutually parallel, the optical axis of the transmitted light T3 and the optical axis of the transmitted light T4 will not be parallel.
As described above, as a result of the collimator lens 114 being disposed so that the space between the first light source 110 and the first collimator lens 114 will be shorter than the focal length of the first collimator lens 114, the laser beam that is headed toward the planar beam splitter 125 will become diverging light, and the front monitor sensor 126 and is disposed to receive the laser beam that is emitted from the first light source 110. Here, the optical axis of the laser beam that passed through the planar beam splitter 125 and is headed toward the front monitor sensor 126 and the optical axis of the laser beam that was internally reflected two or more times in the planar beam splitter 125 and is subsequently headed toward the front monitor sensor 126 will not be mutually parallel. Accordingly, it is possible to inhibit the interference of the laser beam in the effective region of the front monitor sensor 126.
With this kind of conventional optical pickup, the front monitor sensor 126 is able to create an APC signal that is accurately proportionate to the quantity of light of the first light source 110. If this APC signal is fed back to the control unit that is controlling the first light source 110 in order to control the output of the first light source 110, the laser beam can be emitted with a recording power possessing linearity. Thus, upon recording information on an optical disc, the first light source 110 can be accurately controlled so that the laser beam to be emitted will have the intended recording power.
Meanwhile, the second collimator lens 124 is disposed so as to convert the laser beam that is emitted as diverging light from the second light source 120 into diverging light or converging light. Thus, the front monitor sensor 126 is also able to accurately control the second light source 120 so that the laser beam to be emitted will have the intended recording power.
The conventional optical pickup comprising the wedge-shaped beam splitter described in Patent Literature 1 is now explained. The optical pickup comprising the wedge-shaped beam splitter is characterized in comprising a wedge-shaped beam splitter 145 in substitute for the planar beam splitter 125 of the optical pickup 150 shown in FIG. 18.
FIG. 20 is a diagram showing the condition of the transmitted light and the reflected light in the wedge-shaped beam splitter of a conventional optical pickup. With the wedge-shaped beam splitter 145, as shown in FIG. 20, the first surface 145a and the second surface 145b mutually form a predetermined angle. The first collimator lens 114 and/or second collimator lens 124 is disposed so as to converge the laser beam that is emitted as diverging light from the first light source 110 and the second light source 120 into parallel light. With the wedge-shaped beam splitter 145, the angle formed by the first surface 145a and the second surface 145b is decided so that interference will not occur in the effective region of the front monitor sensor 126 due to the internal reflection occurring two or more times.
In FIG. 20, parallel light P5 that is emitted from within the effective region of the first collimator lens 114 enters the wedge-shaped beam splitter 145. Here, the parallel light P5 is split into reflected light R that was reflected by the first surface 145a and transmitted light T5 that passed through the first surface 145a and the second surface 145b and is headed toward the front monitor sensor 126. Parallel light P6 that was emitted from within a different effective region of the first collimator lens 114 is reflected by the second surface 145b after passing through the first surface 145a. In addition, the parallel light P6 that was reflected by the second surface 145b is reflected by the first surface 145a, subsequently passes through the second surface 145b, and is emitted as transmitted light T6.
Here, although the optical axis of the parallel light P5 and the optical axis of the parallel light P6 are mutually parallel, since the first surface 145a and the second surface 145b of the wedge-shaped beam splitter 145 are forming a predetermined angle, the optical axis of the transmitted light T5 and the optical axis of the transmitted light T6 will not be parallel.
As described above, as a result of using the wedge-shaped beam splitter 145 in which the light incident plane and the light emission plane form a predetermined angle, the laser beam that is emitted from the first light source 110, passes through the wedge-shaped beam splitter 145, and is subsequently headed toward the front monitor sensor 126 and the laser beam that is headed toward the front monitor sensor 126 after being internally reflected at least two or more times in the wedge-shaped beam splitter 145 will not be mutually parallel. Accordingly, it is possible to inhibit the interference of the laser beam in the effective region of the front monitor sensor 126 and create an APC signal that is accurately proportionate to the quantity of light of the first light source 110, and accurately control the optical output of the first light source 110.
Moreover, as a result of using the wedge-shaped beam splitter 145, the laser beam that is emitted from the second light source 120, internally reflected once in the wedge-shaped beam splitter 145 and is subsequently headed toward the front monitor sensor 126 and the laser beam that is headed toward the front monitor sensor 126 after being internally reflected at least three or more times in the wedge-shaped beam splitter 145 will not be mutually parallel. Accordingly, it is possible to inhibit the interference of the laser beam in the effective region of the front monitor sensor 126 and create an APC signal that is accurately proportionate to the quantity of light of the second light source 120, and accurately control the optical output of the second light source 120.
Meanwhile, pursuant to the practical application of a blue-violet semiconductor laser, a Blu-ray Disc (hereinafter referred to as “BD”) as a high density, large capacity optical information recording medium (hereinafter also referred as an “optical disc”) of the same size as a CD (Compact Disc) and a DVD (Digital Versatile Disc) has been put into practical use. The BD is an optical disc that uses a blue-violet laser light source for emitting blue-violet light having a wavelength of approximately 400 nm and an objective lens having a numerical aperture (Numerical Aperture: NA) that is approximately 0.85 and records or reproduces information to or from the information recording surface in which the thickness of the light transmitting layer is approximately 0.1 mm.
With a high density optical disc such as a BD, information will be recorded or reproduced to or from a plurality of information recording surfaces. However, since the thickness of the light transmitting layer for each information recording surface is different, third order spherical aberration will occur in accordance with the distance from the optimal light transmitting layer thickness to the information recording surface on the information recording surface that deviates from the optimal light transmitting layer thickness of the objective lens. Incidentally, the optimal light transmitting layer thickness of the objective lens refers to the thickness of the light transmitting layer in which the third order spherical aberration will be minimal when parallel light enters the objective lens. If the wavelength of the laser beam is 400 nm and the NA of the objective lens is 0.85, third order spherical aberration of approximately 100 mλ will occur for a 10 μm thickness deviation of the light transmitting layer. Thus, an optical head for use in this kind of optical disc generally comprises means for correcting the third order spherical aberration.
For example, Patent Literature 2 describes an optical head in which a collimator lens is mounted on a collimator lens actuator, and, in order to negate the third order spherical aberration caused by the thickness deviation of the light transmitting layer, the collimator lens disposed between a light source and an objective lens is moved in the optical axis direction to change the divergence angle or convergent angle of the laser beam that enters the objective lens.
With a BD, following types of discs have been put into practical use; namely, a single layer disc comprising a single information recording surface in which the thickness of the light transmitting layer is 100 μm, and a dual layer disc comprising two information recording surfaces each having a light transmitting layer with a thickness of 100 μm and 75 μm. With an optical disc comprising a plurality of information recording surfaces, since the thickness of the light transmitting layer will differ for each information recording surface, the collimator lens needs to be moved in a wide range in order to negate the third order spherical aberration caused by the thickness deviation of the light transmitting layer or various errors. Specifically, the laser beam that is emitted from the collimator lens will be used in a broad range of the converging light, the parallel light and the diverging light. Accordingly, with an optical head for recording or reproducing information to or from an optical disc including a plurality of information recording surfaces, it is not possible to adopt a configuration using the conventional planar beam splitter, and there is no choice but to use the wedge-shaped beam splitter that entails high production costs.    Patent Literature 1: Japanese Patent Application Laid-open No. 2004-5944    Patent Literature 2: Japanese Patent Application Laid-open No. H11-259906